1. Field of the Invention
The invention relates to a storage battery with several cylindrical cells, the cell walls of which are in close heat-conducting contact with a temperature-control device.
2. Description of the Related Art
Highly loaded battery systems represent an important key component for future hybrid vehicles. High requirements imposed on their power at a relatively low energy content are a characteristic feature of these hybrid vehicle batteries. Typical energy contents in such cases lie in the range from 3 to 10 kWh. The required power during charging and discharging for a typical vehicle in such cases may range up to 100 kW. In addition to pure electrical operation for short distances, mixed operation is also envisioned for hybrid vehicles in which the battery is used as the energy source for improved acceleration. Rapid recharging and high discharge powers are especially necessary during this mode of operation.
The high charging and discharging powers naturally involve high internal losses which accumulate as heat in the battery. The heat losses accumulating in the battery arise in the interior of the cells in the parts through which the charging and discharging current flows. The heat can only be carried off through the surface by radiation, convection, and heat conduction (e.g., through the bottom or the casing surface). In the case of large storage batteries, the ratio of surface to volume is so unfavorable that the heat generated in normal charging and discharging operations can be dissipated only at operating temperatures exceeding values that are detrimental to the service life of the energy storage unit.
The result is a limitation of usefulness due to the necessary cooling pauses, or a reduced service life. In large storage batteries with 50 to 300 cells, a temperature gradient of 5 to 10 Kelvin may arise between the center and the edge of the battery. Because of the different cell temperatures then, the fully charged state is achieved at different times due to the temperature dependence of the charge absorption and other processes such as hydrogen evolution and oxygen recombination in water-based battery systems. In the case of a limited charging time, this leads to discrepancies between the charge states of the individual cells and ultimately to their failure after the overloading of cells with low charge states. This is also true of storage batteries for purely electrically driven road vehicles.
Because of the voltage position favored by automobile manufacturers of 300 to 400 volts, a storage battery based on a nickel-hydride system must consist of about 300 individual cells. Depending on the required energy contents of the storage battery, therefore, cells with a capacity between 7 and 25 Ah are required. Cells with such a capacity are preferably manufactured as cylindrical cells because the manufacturing cost for the production of such cells is relatively low. A storage battery is described in German patent no. DE-A 4326943. In this case, the cells are arranged in a rectangular box in such a way that the end faces of the cells are exposed. A stream of air is provided as cooling which utilizes the channels formed by the wedge-shaped hollow spaces between the cells. Primarily because of the previously described internal cells losses, such a storage battery in a typical high-load operation experiences rapid heating which is difficult to counteract by ordinary air cooling. One of the reasons for this is the very low heat capacity of air at 1.01 kJ/kg*K. A typical cyclic operation, e.g., of a storage battery consisting of 300 cells each with a capacity of 10 Ah, looks as follows:
Charge 1: 100 A Charge about 44 kW Charge 61 Wh (5 sec) power: energy: Charge 2: 30 A Charge about 13 kW Charge 56 Wh (15.5 sec) power: energy: Discharge: 120 A Discharge about 42 kW Discharge 93 Wh (8 sec) power: energy: Pause: 31.5 sec Loss about 1600 W power:
The energy difference of about 22% between charging and discharging is caused by the different voltage levels. These are a consequence of ohmic losses during cell operation and the hysteresis in the case of a single potential characteristic of the positive nickel-hydroxide electrode.
FIG. 4 shows that a battery cooled with air of 20.degree. C. in the case of 80 minutes of operating time is heated up to values above 60.degree. C., in which case, depending on the cell size and the arrangement of the individual cells (close air inlet or outlet), large temperature differences occur between the individual cells.
Liquid cooling of the cells is known for storage batteries constructed from prismatic cells. Thus, for example, in German utility patent G 92 10 384, plate-shaped liquid-permeated cooling bodies are disclosed which, in each case, are arranged between the large side faces of neighboring cells. The storage battery, consisting of the individual cells and the cooling bodies positioned between them, is held together by a battery trough. Such cells represent the preferred type for traction batteries. Plate-shaped cooling bodies are problematic for cylindrical cells because of the wedge-shaped volume remaining in that case. The advantages of pressure-stable and easily manufactured cylindrical cells are opposed by the disadvantages that the wedge-shaped volume between the cylindrical cells cannot be utilized electrochemically, and the volume and with it the weight of the temperature-regulating fluid used increases so that the gravimetric energy density of the storage battery is reduced.